An inertial guidance system typically uses gyroscopes and accelerometers to continuously monitor acceleration to which a body is subjected, with respect to three orthogonal axes. Using well-known laws of physics, the body's position and heading can be determined at any point in time from the acceleration data, based upon knowledge of the initial position of the body and the time over which the acceleration occurs. However, even a brief interruption in the continuity of velocity data can greatly affect the accuracy of the system in determining the position and heading of the body.
Certain types of accelerometers used in inertial guidance systems are characteristically resistant to loss of velocity data during brief interruptions in the processing of their output signals. For example, a specific force integrating resolver (SFIR) type accelerometer relies on the principle of gyroscopic precession to measure changes in velocity of an attached body. A spinning body tends to preserve its angular momentum, and therefore rate of precession is unaffected during a brief power interruption. Although SFIR accelerometers are very accurate, due to their mechanically complex design, they are relatively expensive. Also, because of their extensive use of moving parts that tend to wear out with continued use, the operational life span of these devices is shorter than accelerometers using piezoelectric crystal acceleration sensors, such as a vibrating beam accelerometer.
Vibrating beam accelerometers typically include two quartz crystals that are each mounted between a supporting frame and a proof mass. The proof mass is suspended from the supporting frame by a flexure hinge that allows the proof mass to deflect freely along the direction of acceleration, yet firmly supports it in two other orthogonal directions. The quartz crystals are driven into resonance, producing a signal indicative of the acceleration acting on a body to which the accelerometer is attached. Acceleration acts on the proof mass, causing the frequency of vibration of the crystals to change in proportion to the acceleration. Generally, one of the quartz crystals is mounted so that it experiences a compression force, while the other experiences a tension force in response to a given acceleration acting on the proof mass along a sensitive axis of the accelerometer.
Continued development of vibrating beam accelerometers has improved their accuracy to a point that they are now being considered as replacements for SFIR accelerometers in critical guidance systems. However, unlike SFIR accelerometers, vibrating beam accelerometers do not include a spinning mass that inherently retains its angular momentum during a short-term interruption of their output signal.
A method for using a vibrating beam accelerometer to provide a velocity storage capability is disclosed in commonly assigned U.S. Pat. No. 4,712,427. The method for storing and recovering data describing a body's change in velocity disclosed in that patent uses the sum and differences in phase of signals produced by the two quartz crystals over an unknown interval of time spanning a brief loss in electrical power supplied to the navigation system including the system clock and the accelerometer. This method is limited to a velocity storage/recovery of about 0.005 to 0.01 g-secs. U.S. Pat. No. 4,712,427 also describes a method to increase the range of recoverable velocity change. However, the technique requires the use of four force-sensing crystals, which is equivalent to employing two accelerometers to measure acceleration along one axis. Clearly, it is preferable to use a single accelerometer (two force-sensitive crystals) for this purpose. In addition, certain applications require computation of velocity change at least two orders of magnitude greater than the range possible with the technique of U.S. Pat. No. 4,712,427. It is thus an object of the present invention to provide these advantages.